In general, the wrist mechanism of industrial robots is the front-end mechanism in which robot operations should be integrated, so that not only revolution and rotation of what is called ".alpha.", ".beta.", and ".gamma." axes be universal, but also the mechanism be small in size, smooth in rotational transmission and strong in force. One of the wrist mechanisms for an industrial robot according to the prior art is disclosed by the Laid-Open Japanese Patent Application No. 1988-185595. The side sectional view of the prior art is shown in FIG. 3.
In FIG. 3, the outer rotation shaft 7 is supported by the bearing 51 within the robot arm 63 (".gamma." axis) whose speed is reduced by the speed reducer (not shown), and further the inner rotation shaft 8 is supported by the other bearing within the-outer rotation shaft 7. One output power transmitted by the outer rotation shaft 7 enters the speed reducer 6 on the ".beta." axis via the bevel gears 5a and 5b to make up and down rocking (what is called ".beta." rotation), while the other output power transmitted by the inner rotation shaft 8 enters the speed reducer 11 on the ".alpha." axis via the bevel gears 4a and 4b, the spur gear train 9a, 9b and 9c and the bevel gears 2a and 2b to make what is called ".alpha." rotation of the wrist 1.
In the wrist mechanism for an industrial robot according to the prior art, however, the speed reducer 11 for ".alpha." rotation is arranged on the member 10 at the front end portion of the wrist, and the speed reducer 6 for ".beta." rotation is arranged close to the inner rotation shaft (8), thus both constituting the last stage of the respective power transmission systems: In more detail, one output power transmitted enters the speed reducer 11 on the ".alpha." axis via the bevel gears 4a and 4b, the spur gear train 9a, 9b and 9c and the bevel gears 2a and 2b to make the ".alpha." rotation of the wrist 1, while the other output power transmitted enters the speed reducer 6 via the bevel gears 5a and 5b to make the ".beta." rotation. With this configuration, the gear train 9a, 9b and 9c necessitates the idle spur gear 9b for adjusting potential backlash taking place between the spur gears 9a and 9c in an assumed direct engagement of them, thus requiring many pieces of rotating parts, resulting in a large moment of inertia.